CREATE TABLE test_trigger (
  id serial primary key,
  t text
);
CREATE OR REPLACE FUNCTION test_trigger_str(r test_trigger) RETURNS text AS $_$
BEGIN
  return 'hello '||r.t;
END;
$_$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION ftrig_test_trigger() RETURNS trigger AS $_$
BEGIN
  NEW.t := test_trigger_str(NEW);
  return NEW;
END;
$_$ LANGUAGE plpgsql;

CREATE TRIGGER test_trigger_bi BEFORE INSERT
  ON test_trigger FOR EACH ROW
  EXECUTE PROCEDURE ftrig_test_trigger();
  
insert into test_trigger(t) values ('world');
select * from test_trigger;

 id |      t
----+-------------
   1  | hello world